Olfactory receptor neurons are directly exposed to the environment and as a consequence, the olfactory nerve serves as a direct portal of entry into the central nervous system (CNS) for viruses and toxins. Spread to distant sites in the brain and spinal cord occurs via the secondary and tertiary connections of this nerve. Mouse hepatitis virus, strain JHM (MHV-JHM), has a strong tropism for the olfactory system and should be useful for answering many questions about the spread of virus within this system. A model has been developed using intranasal and intrabulbar inoculation of MHV-JHM in suckling and 4-6 week old C57BL/6 mice. The data show that MHV-JHM spreads to many of the CNS sites with known transneuronal connections to the olfactory bulb, but does not infect other structures with similar connections, such as the locus coeruleus. In marked contrast, herpes simplex virus I, another neurotropic virus, appears to infect a different subset of the structures connected to the olfactory bulb after the same routes of inoculation. Viral spread to only subset of the structures connected to the olfactory bulb may reflect the presence of the cellular receptor for MHV-JHM on susceptible cells and its absence on resistant cells. The specific objectives which will be addressed in this research plan are: 1) To compare in more detail the transneuronal pathways followed by HSV I and MHV-JHM. Intranasal and intracerebral inoculation of virus with analysis by in situ hybridization will be used in these experiments. 2) To determine whether the differences in uptake of MHV-JHM and HSV I by the locus coeruleus, piriform cortex, and the insular cortex and its connections are attributable to anatomical differences in the transynaptic movement of these viruses. These experiments will be performed using immunocytochemistry for detection of specific neuro- transmitters and for analysis of retrograde transynaptic movement, and in situ hybridization for identification of cells infected with virus. 3) To isolate the cellular receptor for MHV-JHM within the murine CNS and compare its distribution with the location of MHV-infected cells. The receptor will be identified using protein chemistry and DNA recombinant technology, and its distribution within the CNS determined by immunocytochemistry and in situ hybridization..